The present invention relates to an improved sensing device for use in an electronic draft control system of the type disclosed in L. G. Schowalter U.S. Pat. No. 3,516,498, issued June 23, 1970 and assigned to the assignee of the present invention.
One of the most common present day systems for connecting an implement to a tractor is known as a three-point hitch. The three-point hitch consists of a pair of draft arms that provide the connection between the implement and the tractor and a rock arm connected to a rock shaft and also to the draft arms through links. The rock shaft is positioned by a hydraulic motor. Fluid is selectively admitted to the hydraulic motor through a valve which is actuated in response to changes in the draft forces on the draft arms, the manual positional setting of the system and the position of the rock shaft. Heretofore, most of the systems that are commercially available consist of a strictly mechanical sensing system for sensing changes in draft load, the position of the rock shaft and the position of a manual control lever. These mechanical systems become extremely complicated and require a substantial amount of maintenance.
As such, many recent efforts have been directed toward development of an electronic system for controlling the hydraulic valve which supplies fluid to the hydraulic motor. Examples of such devices are shown in U.S. Pat. No. 3,814,188, issued on June 4, 1974, as well as the above mentioned Schowalter patent.
So far as presently known, most systems that have been proposed rely primarily upon signal amplification for measuring changes in the various parameters of the system. In other words, a signal is developed from the various sensing devices, and the signals are combined and amplified to produce an output signal. When the combined output signal exceeds a certain level, a control valve is actuated to make a correction. However, these systems have lacked reliability because presently known sensing devices do not have the requisite sensitivity to measure small changes in the respective parameters.
One of the problems encountered with the development of a completely electronic control system for controlling the position of draft arms of an implement deals with the sensing device that is utilized for sensing the changes in position of the various components in the system. It will be appreciated that in order to have a reliable system, the sensing device must be extremely durable in order to be capable of absorbing the vibrations that are developed in the vehicle and the implement and yet be sensitive enough to be able to sense changes of small magnitude.